A staff member is performing a stock inventory with a mobile UHF RFID reader. Everything flows smoothly until the mobile UHF RFID reader notes that there is another UHF RFID reader in the DC – a fixed one. What happens – do they both freak out?
Authors: Toni Heijari & Suvi
The Tech Geek will tell you.
Shortly, what happens is that the readers detect each other.
This is called the RFID reader collision problem. By using
the term 'reader collision' we define it as a phenomenon where an
interrogation signal from a specific reader interferences the
operation of a tag or another reader. This sort of a problem is
augmented particularly in a dense reading environment, for example
in a warehouse where multiple readers are operating in the same
But where does the problem arise?
The channels used earlier in the EU region were defined by EN
302 208 or EN 300 220, and were allocated in a way that
there were 10 high power channels. Also, the LBT system (Listen
Before Talk) was in use. The idea was that a reader would
listen if the channel is free before it transmits signal - also the
tag would respond in the same channel. This method works quite
nicely if the number of readers is limited to only a few readers.
The threshold for specific channel was low in such a way that in
order to receive permission to use that channel it was required
that there were no other readers operating in a certain channel for
about 1 km radius (when operating at maximum output power). In
other words, you could say that there could be only 10 readers
operating in the area of 1 km radius without any reduction in
Another problem is that when Reader A is reading a specific tag
population and Reader B is operating in close proximity, the tags
in question can interfere from the Reader B signal. This is due to
the reason that the tags have no channel selectivity. This in turn
means that they interoperate only the power they are receiving. As
the stronger interrogation signal will run over the weaker one,
only one reader can read the specific tag population at a time as a
Introducing the DRM
When the reader collision problem with multiple readers using
LBT was acknowledged, the ETSI (European Telecommunications
Standards Institute) introduced the new channel plan and
method for multiple readers to operate in a dense reading
environment. This method was called DRM (Dense Reader
Mode).The number of high power channels in which readers
operate was limited to 4. The free channels, which were dedicated
to tag replays, were left between those channels. The Miller
sub-carrier encoding and link frequencies of 200 - 400 kHz were
adopted into use in order to get the tag replay away from the
channel the reader was using. Generally, link frequency defines the
spacing between the reader transmission frequency and the tag
replay frequency. The order of the Miller encoding (M2/M4/M8)
impacts on the spectral density of the tag replay concentrating
around the link frequency offset. The ETSI channel plan is
introduced in Picture 1.
Picture 1. ETSI channel plan (EN
302 208 v1.4.1).
How to avoid the collision
As the technology constantly develops, certain types of problems
are already been avoided. When adapting the new channel plan and
the DRM into use, the reader is no longer required to use the LBT
technique. This will increase the reading speed since the reader
doesn't have to listen every time before it transmits. Moreover,
now a single reader will no longer reserve the channel for itself
in a large surrounding area. Also the tag replays, which are 40 -
120 dB lower than the readers' signals, are not blocked because of
the dedicated tag replay channels.
But there are still some limitations. The performance of the DRM
filters inside the reader and the overall robustness of the
receiver design will now limit the performance when readers are
operating in a close proximity. Also the used channels and the
directivity of the reader antenna will affect the performance
because they greatly determine the isolation between the two
The used channels and the antenna radiation pattern issues will
be further discussed when the Tech Geek returns with Several
UHF RFID readers in the DC, part 2. In fixed installations a
good practice is not to put two readers equipped with directive
antennas facing toward each other, or in that case the reading
should be sequential.
To be continued…
ETSI 302 208 v1.4.1 specification